You're probably thinking that this is a YouTube problem... maybe it is, but I have only found this problem on IE10 and any other browser works fine.
When I go to a video page (not home page or search results) on YouTube, I always get the iconic 'monkey' error.
For example, this link will cause an error: http://www.youtube.com/watch?v=9bLzEtfsxhM
The error information changes every time. I've tried setting the document mode to IE9 but it doesn't work. I've got another computer at home that runs IE9 and the videos work fine. I've tried changing the user agent string but this doesn't work either. Refreshing cache, editing request in Fiddler, etc., doesn't work.